1. Field of the Invention
The invention relates to a lock device and an electric power steering system.
2. Description of Related Art
Conventional lock devices are described in, for example, Japanese Patent Application Publication No. 2001-219816 (JP 2001-219816) and Japanese Patent Application Publication No. 2002-308049 (JP 2002-308049).
JP 2001-219816 describes a column-type electric power steering system provided with a steering lock device. An engagement portion is formed at a worm shaft side (input side), the worm shaft transmitting the rotation of an electric motor to a speed reducer, or at an electric motor output shaft side. A locked state is achieved by inserting a lock member into the engagement portion, and an unlocked state is achieved by removing the lock member from the engagement portion.
The lock member is advanced or retracted with the use of an elastic member that urges (advances) the lock member toward the engagement portion and an actuator that attracts the lock member to remove (retract) the lock member from engagement portion. The lock member is advanced or retracted in the following manner. When an ignition key is turned on, electric current application to a solenoid that serves as the actuator and that is fixed to a casing is controlled. Thus, an attraction force that counteracts an urging force generated by the elastic member is generated to retract the lock member provided with a moving core so that the lock member is removed from the engagement portion. As a result, the unlocked state is achieved. When the ignition key is turned off, attraction of the moving core by the solenoid is stopped, and the lock member is advanced toward the engagement portion by an urging force generated by the elastic member. As a result, the lock member is engaged with the engagement portion, whereby the locked state is achieved.
As described above, the locked state and the unlocked state are achieved at the electric motor output shaft side or at the worm shaft side that is the input side of the speed reducer (i.e., at a stage prior to output of assist torque based on the torque applied to a steering wheel). Thus, in the locked state where the lock member is engaged with the engagement portion, a large force based on the torque applied to the steering wheel is no longer applied directly to the lock member, which enables downsizing of the lock member.
However, when the ignition switch is on, electric current is applied to a coil to retract the lock member from the engagement portion. Therefore, if, for example, breakage of a harness, disconnection of a connector, or an instantaneous reduction in battery voltage occurs, electric current application to the coil is stopped. As a result, the lock member is advanced and engaged with the engagement portion due to an urging force generated by the elastic member. In some cases, the steering wheel may be locked while a vehicle is traveling. With this regards, there is still room for improvement.
JP 2002-308049 describes a structure in which a key portion and a lock mechanism portion of a steering shaft are unitized so as to be mechanically linked to each other. In the structure, a cam member that rotates together with a key rotor is provided between the key rotor and the lock mechanism portion of the steering shaft and extends to the lock mechanism portion coaxially with the key rotor. A locking lever that is linked to insertion and removal of the key is provided. When the key is turned from ACC position to LOCK position, the cam member is operated. In accordance with the operation of the cam member, the lock member that is provided at the lock mechanism on the steering shaft side is operated and is brought to a state where the lock member can be locked with the steering shaft. When the key is removed from LOCK position, the locking lever is operated. In accordance with the operation of the locking lever, the lock member is operated to be inserted in a groove of the steering shaft. Thus, the locked state is achieved.
JP 2002-308049 A describes the structure in which the key portion and the lock mechanism portion of the steering shaft are unitized so as to be mechanically liked to each other. Therefore, if the key portion is provided at an instrument panel at a driver's seat, the lock mechanism portion is located in front of the knee of a driver, which may impose restrictions on the strength and installation position of the lock mechanism portion.
In order to address this problem, the following configuration may be employed. An operation portion such as a key device and an actuator portion such as a lock mechanism are separated from each other. A lock member at the lock mechanism is moved to the lock position by a spring member. When the key is inserted and turned to ACC position (when locking is cancelled), the fact that the key is turned to ACC position is detected by, for example, detection unit, and drive unit such as a motor is driven based on a detection signal to move the lock member to the locking cancellation position.
A device is required which maintains the locking cancellation state so that the locking operation is not erroneously performed in the locking cancellation state where locking by the lock member is cancelled. Conventionally, the key portion and the lock mechanism portion of the steering shaft are mechanically linked to each other. Therefore, as long as the key rotor is at a predetermined rotation position, the locking cancellation state where locking by the lock member is cancelled is maintained by the cam member.
In the above-described structure where the operation portion such as the key device and the actuator portion such as the lock mechanism are separated from each other, there is no cam member. Accordingly, it is necessary to provide a device that maintains the locking cancellation state, at the actuator portion. For example, a locking cancellation maintaining member is attached to a plunger of a solenoid, which is an electric drive unit. An electric signal is generated based on the operation of the operation portion, the solenoid is driven according to the electric signal, and the locking cancellation state in which locking by the lock member is cancelled is maintained by the locking cancellation maintaining member. However, in the structure in which the locking cancellation maintaining member is operated by electric drive unit such as a solenoid, malfunction due to an electrical problem (e.g., breakage of a harness, disconnection of a connector, or an instantaneous reduction in battery voltage) may occur. In this regard, there is still room for improvement.